Product Billing Donation

aMember has optional plugin that allow you to accept variable amount from your customers.

This plugin add new Form Brick - Donation, this brick can be used on sign up form
either instead of or in addition to Product Brick. This brick allow customer to choose
amount to pay on his own, also customer can choose to make donation recurring.

You can add this brick to any of your existing sign up forms or create separate form for it
aMember CP -> Configuration -> Forms Editor -> (edit)

You can set product and other settings within brick configuration.

This brick allow you to use two standard layouts:

Default - input box where user can specify amount

Emotional - slider with smile - more amount of donation means more happiness in smile

Obviously it is not required to use this plugin only for donation. You can use
it in order to handle any situation when customer should be able to specify amount of payment
on his own. All titles/labels can be changed in brick configuration to better suit your use case.